1. Field of the Invention
The present invention relates to a liquid crystal composition suitable for use mainly in an active matrix (AM) element, and an AM element comprising the composition.
2. Description of the Related Art
A liquid crystal display element is classified, based on an operating mode of liquid crystals, into phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and the like. It is classified into passive matrix (PM) and active matrix (AM) based on a driving mode. PM is further classified into static, multiplex and the like, and AM is classified into thin film transistor (TFT), metal insular metal (MIM) and the like. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. The liquid crystal display element is classified, based on a light source, into a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both of them.
These elements comprise a liquid crystal composition having suitable characteristics. To improve the general characteristics of the composition can lead to the production of an AM element having good general characteristics. The general characteristics of the composition are related to those of an AM element as summarized in Table 1. The general characteristics of the composition shall be further explained based on an AM element on the market. The temperature range of a nematic phase is related to the temperature range in which the element can be used. The desirable higher limit temperature of a nematic phase is 70° C. or more and the desirable lower limit is −20° C. or less. The viscosity of the composition correlates with the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, the composition desirably has a low viscosity, and a low viscosity at low temperature is more desirable.
TABLE 1General characteristics of a liquid crystal compositionand an AM elementGeneral Characteristics ofGeneral Characteristics of anNo.the CompositionAM Element1Wide temperature range ofAvailable in wide temperaturea nematic phaserange2Low in viscosity1)Short in response time3Suitable optical anisotropyHigh in contrast ratio4Large in positive or negativeLow in operating electricdielectric anisotropy2)voltage5High in specific resistanceHigh in voltage holding ratioand high contrast ratio1)The liquid crystal composition can be injected into a cell in a short time. 2)The liquid crystal composition contributes to high aperture ratio in an IPS element. 
The optical anisotropy of the composition correlates with the contrast ratio of the element. To attain a maximum contrast ratio of the element, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) should be designed to be approximately 0.45 μm. Accordingly, the optical anisotropy of the composition is in the range of 0.08 to 0.12. A large dielectric anisotropy of the composition contributes to low operating electric voltage of the element. Accordingly, a large dielectric anisotropy is desirable. High specific resistance of the composition contributes to a high voltage holding ratio and a high contrast ratio of the element. Accordingly, a liquid crystal composition having a high specific resistance in the initial stage is desirable. Further, it is desirable that the composition still has a high specific resistance even after a long-term use.
It is desirable that the composition suitable for an AM element of a TN mode has a high specific resistance. In contrast, the composition suitable for an AM element of an IPS mode may have a low specific resistance in comparison to the composition suitable for an AM element of a TN mode. This is because the AM element of an IPS mode which comprises the composition having a low specific resistance has a high voltage holding ratio. This is explained in IDW'97 Proceedings of The Fourth International Display Workshops p171-p174. The composition suitable for an AM element of an IPS mode may comprise a compound having cyano at the end of the molecule.
In an AM element of an IPS mode, a large dielectric anisotropy of the composition contributes to high aperture ratio of the element. Accordingly, a composition having a low viscosity and a large dielectric anisotropy is specifically desired for the element so that it displays an animated cartoon at a high aperture ratio. The conventional compositions are disclosed in the following patent documents; JP10-204016A/1998(U.S. Pat. No. 6,007,740, JP10-204436A/1998(U.S. Pat. No. 5,961,881), JP2001-003051A (U.S. Pat. No. 6,325,949), JP2001-003053A (U.S. Pat. No. 6,315,922), JP2001-123170A (U.S. Pat. No. 6,497,929), and WO01/046336A.